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This diagram shows the formula: Total monthly cost in pounds equals 20, then open parenthesis, the total number of minutes used minus 300, close parenthesis, multiplied by 0.4. A bubble points to the 20 and says ‘This is the monthly charge of £20.’ A bubble points to the bracket and says ‘The expression in the brackets works out the extra minutes that have been used.’ A bubble points to the 0.4 and says ‘This is the cost of each extra minute in £. 40p = £0.4.’